Records of the Free Vibrations of a Tuning-fork. 445 
the battery. When the circuit of battery B is closed, the re- 
lay-armature makes contact at CU’. From ©’ a wire passes to 
the framework of a De la Tour’s siren S. Attached to the 
frame of the siren is an insulated support carrying a platinum 
spring K, which bears against an ebonite drum on the axis of 
the siren, and touches at each revolution a strip of metal em- 
bedded in the drum, and in electrical communication with the 
axis. From the spring, a wire passes to the platinum pen G. 
3) 
a 
co} 
see. 
cE 
{ 
4 
S 
|| 
OU a cance 
[t follows that, when the armature is in contact with C’, which 
is about +°%; of each second, the current of the battery B’ flows 
through the post D, the armature A, the contact point O° and 
the siren S to the pen G, and so to the battery. A clock-work 
gives motion to a fillet of paper moistened with a solution of 
lodide of potassium, drawing it between the pens and the 
